Hi, I tried logging in today and had the same message, and then I went and checked and it said I didn't have dawntrail, even though I've been playing early access lol.

I went to square enix web site > orders > copied my code > went to mogstation > **ADDITIONAL EXPANSIONS** > and put my code in (again) and it worked for me. I am able to log in finally. IDK why it said I didn't already register it when I did before, but I just had to do it again.
Hope this helps.